diff --stat: add a test for output with COLUMNS=40

diff --stat: add a test for output with COLUMNS=40

In preparation for the introduction on the limit of the width of the
graph part, a new test with COLUMNS=40 is added to check that the
environment variable influences diff, show, log, but not format-patch.
A new test is added because limiting the graph part makes COLUMNS=200
stop influencing diff --stat behaviour, which isn't wide enough now.
The old test with COLUMNS=200 is retained to check for regressions.

Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>